In a manufacturing process of a semiconductor device or a FPD (Flat Panel Display), plasma is used to perform a process, such as etching, deposition, oxidation, or sputtering, to perform a good reaction of a processing gas at a relatively low temperature. Conventionally, plasma generated by a high frequency electric discharge in MHz frequency band has been used in this kind of plasma process. The plasma generated by the high frequency electric discharge is largely divided into capacitively coupled plasma and inductively coupled plasma according to a plasma generation method (in view of an apparatus).
Generally, in an inductively coupled plasma processing apparatus, at least a part (for example, a ceiling) of walls of a processing chamber may have a dielectric window, and a high frequency power is supplied to a coil-shaped RF antenna positioned at an outside of this dielectric window. The processing chamber is a depressurizable vacuum chamber, and a processing target substrate (for example, a semiconductor wafer or a glass substrate) is provided at a central region within the chamber. A processing gas is supplied into a processing space formed between the dielectric window and the substrate. A high frequency AC magnetic field having magnetic force lines is generated around the RF antenna by a high frequency current flowing in the RF antenna. The magnetic force lines of the high frequency AC magnetic field are transmitted to the processing space within the chamber via the dielectric window. As the RF magnetic field of the high frequency AC magnetic field changes with time, an inductive electric field is generated in an azimuth direction within the processing space. Then, electrons accelerated by this inductive electric field in the azimuth direction collide with molecules or atoms of the processing gas to be ionized. In this process, donut-shaped plasma may be generated.
Since a large processing space is formed within the chamber, the donut-shaped plasma can be diffused efficiently in all directions (especially, in a radial direction) and a plasma density on the substrate becomes very uniform. However, only with a conventional RF antenna, the plasma density on a substrate is not sufficiently uniform for most plasma processes. In the plasma process, it is also one of the important issues to improve uniformity or controllability of a plasma density on a substrate since a uniformity/reproducibility and a production yield of a plasma process depend on the plasma uniformity or controllability.
In the inductively coupled plasma processing apparatus, a characteristic (profile) of a plasma density distribution within the donut-shaped plasma formed in the vicinity of the dielectric window within the chamber is important. Especially, the profile of the plasma density distribution affects characteristics (especially, uniformity) of a plasma density distribution on the substrate after the diffusion of the plasma.
In this regard, there have been proposed several methods for improving uniformity of a plasma density distribution in a diametrical direction by dividing the RF antennal into a multiple number of circular ring-shaped coils having different diameters. There are two types of RF antenna division methods: a first type of connecting the multiple number of circular ring-shaped coils in series (see, for example, Patent Document 1) and a second type of connecting the multiple number of circular ring-shaped coils in parallel (see, for example, Patent Document 2).    Patent Document 1: U.S. Pat. No. 5,800,619    Patent Document 2: U.S. Pat. No. 6,164,241
In accordance with the first type method among the aforementioned conventional RF antenna division methods, since an entire coil length of the RF antenna is large as a sum of all the coils, a voltage drop within the RF antenna may be fairly large and not negligible. Further, due to a wavelength effect, a standing wave of electric current having a node in the vicinity of a RF input terminal of the RF antenna may be easily formed. For these reasons, in accordance with this first type method, it may be difficult to achieve uniformity of a plasma density distribution in a diametrical direction as well as in a circumferential direction. Thus, the first type method is essentially deemed to be inadequate for a plasma process for which plasma of a large diameter is necessary.
Meanwhile, in accordance with the second type method, the RF currents supplied to the RF antenna from a high frequency power supply flows through an inner coil having a small coil diameter (i.e., smaller impedance) in a relatively large amount, whereas a relatively small amount of RF current flows through an outer coil having a large diameter (i.e., larger impedance). Accordingly, a plasma density within the chamber may be high at a central portion of the chamber in the diametrical direction while the plasma density may be low at a periphery portion thereof. Thus, in the second type method, variable capacitors for adjusting impedance are additionally added (connected) to respective coils within the RF antenna to adjust a ratio of the RF currents flowing through the respective coils. However, there is a limit in a variable range of the RF current ratio. Accordingly, it has been difficult to precisely control a plasma density distribution in the vicinity of a substrate held on a substrate holding unit.